/Engine/Source/Runtime/Engine/Public/Net/NetPacketNotify.h
Write NotificationHeader, and update outgoing ack record if bRefresh is true we will attempt to refresh a previously written header if the resulting size will be the same as the already written header. returns true if data was written, and false if no data was written which might be the case if we try to rewrite an existing header but the required size differs.
bool WriteHeader ( [FBitWriter](API\Runtime\Core\Serialization\FBitWriter) & Writer, bool bRefresh )